This invention relates to an improved method of and means for controlling the height of a plasma arc cutting torch above a workpiece, and more particularly to such a method and means specifically designed to automatically compensate for variations in travel speed of the arc torch above the workpiece during actual operation.
The use of plasma arc torches to cut metal workpieces to a desired shape is known in the art. Thus, one or more plasma arc torches are mounted over a work table which supports the workpiece and appropriate apparatus is provided for moving the torch or torches over the surface of the workpiece in a plane generally parallel thereto to enable the cutting of the workpiece to the desired shape.
However, in such apparatus, movement of the torch or torches along the desired path in a plane parallel to the workpiece is normally controlled by a tape program which must take into account both the configuration of the cut and the mechanical limitations of the apparatus. In order for the torch to follow the desired path, its drive mechanism must be controlled in both the X axis and the Y axis of the plane parallel to the workpiece, causing the torch to move in a given vector and at a given velocity dependent upon the relative speed of the X axis and the Y axis drives.
When the given velocity of the torch with respect to the workpiece is high, as it would be when cutting thinner plates, mechanical considerations tend to require that such velocity be substantially reduced when negotiating sharp turns since very large forces tend to be associated with an abrupt vector change resulting in a deflection of the torch from the desired path. Thus, if the velocity is not reduced, the cut would tend to be inaccurate or the torch drive components would tend to be overstressed. Even in plasma arc cutting apparatus with mechanically sturdy drive components, long periods of use and resultant wearing or drive components will tend to cause the torch to overshoot the desired cutting path during sharp turns if velocity is not reduced. Accordingly, the velocity of the torch with respect to the workpiece must be controlled with the limitations of the particular apparatus in mind in order to maintain the desired cutting path.
The prior art discloses systems in which signals representative of arc voltage and arc current are utilized to maintain a preselected spacing between the torch and workpiece by moving the torch toward or away from the workpiece (i.e. along a Z axis perpendicular to the plane parallel to the workpiece). Thus, where the torch is at the desired spacing, the voltage and current signals are representative of the proper spacing and the torch will not move closer to or further from the surface of the workpiece during a cutting operation so long as the velocity at which the torch is moved over the surface of the workpiece is not reduced below that which is just sufficient to enable the cutting of the workpiece.
The result of reducing the vector velocity of the torch in negotiating sharp turns is that new material will be presented to the arc at a reduced rate. When this occurs, molten material will be blown clear of the cut and the arc can blossom through the cut permitting the arc to extend to the work table underlying the workpiece. The resultant increase in arc length will increase arc resistance thus reducing arc current and increasing arc voltage. These conditions would be interpreted by the control systems of the prior art as an indication of excessive torch-to-workpiece spacing and the torch would tend to be driven toward the workpiece. Damage to the torch or the workpiece or both would result but for safety devices which may be built into the system to inactivate it where an excessive change in torch-to-workpiece spacing is called for.